1. Field of the Invention
This invention relates to hydrophobic inorganic fine particles used in image forming processes such as electrophotography, electrostatic printing and toner jetting, a process for producing the hydrophobic inorganic fine particles, and a toner containing the hydrophobic inorganic fine particles.
2. Related Background Art
It is commonly known to add small-particle-diameter inorganic fine particles to toner particles for the purpose of controlling toner characteristics such as chargeability and fluidity of toners to achieve good developing performance.
In toners to which such small-particle-diameter inorganic fine particles are added, it has been ascertained that the small-particle-diameter inorganic fine particles come to stand buried in surface portions of toner particles because of a stress applied between a toner and a carrier when the toner is used as a two-component developer, a stress applied from a developing blade and a developing sleeve when the toner is used as a one-component developer, an impact against inner walls of a developing assembly and a toner agitation blade, and a mutual impact between toner particles.
In order to make the small-particle-diameter inorganic fine particles less buried, it is effective to use large-particle-diameter inorganic fine particles in combination, as disclosed in Japanese Patent Applications Laid-Open No. H04-204751, No. H05-346682, No. H06-313980, No. H06-332235 and No. H07-92724.
The large-particle-diameter inorganic fine particles have an effect as a spacer, and hence they prevent toner particle surfaces to which the small-particle-diameter inorganic fine particles have adhered, from coming into direct contact with the carrier, developing blade, developing sleeve, developing assembly inner walls, toner agitation member and other toner to lessen the stresses. This makes the small-particle-diameter inorganic fine particles kept from being buried in the surface portions of toner particle, and brings achievement of longer lifetime of toners.
Japanese Patent Application Laid-Open No. H04-204751 discloses a toner having hydrophobic fine silica particles and hydrophobic fine titanium oxide particles or hydrophobic fine aluminum oxide particles, which is a toner characterized in that the hydrophobic fine titanium oxide particles or the hydrophobic fine aluminum oxide particles have peaks at 10 to 20 μm and 30 to 60 μm in primary particle diameter.
Japanese Patent Application Laid-open No. H05-346682 discloses a toner characterized in that an inorganic fine powder having a BET specific surface area of less than 80 m2/g and treated with a silicone oil and an inorganic fine powder having a BET specific surface area of 80 m2/g or more and treated with a silane coupling agent.
Japanese Patent Application Laid-open No. H06-332235 discloses a toner for electrophotography comprising toner particles and at least two types of external additives, which is a toner for electrophotography which is characterized in that particles of 5 μm or smaller are present in a proportion of 1 to 8% by volume in particle size distribution of toner particles, that a first external additive has an average particle diameter of 0.1 to 0.5 μm in number base of primary particles, and that a second external additive has an average particle diameter of 20 nm or less in number base of primary particles and is hydrophobic.
Japanese Patent Application Laid-Open No. H07-104501 discloses a proposal of a toner making use of hydrophobic fine silica particles of 15 to 20 nm in particle diameter and hydrophobic fine silica particles or alumina fine particles of 13 nm or less in particle diameter.
However, because of the addition of two types of hydrophobic inorganic fine particles different in particle diameter, these toners have had problems in respect of mixability of the both and dispersion on the surfaces of toner particles, and had insufficient development durability and charging stability.
Japanese Patent Application Laid-Open No. H06-313980 discloses a developer characterized in that inorganic fine particles have, in its number primary particle diameter distribution curve, i) a maximum value of number proportion at each of a primary particle diameter x (nm) (where 20≦x≦50) and a primary particle diameter y (nm) (where 3x≦y≦6x) and ii) 10% by number or less of number proportion in the primary particle diameter (x+y)/2 (nm), have a value of X/Y within the range of from 0.5 to 2.0 where the number proportion of inorganic fine particles on the side of small particle diameter which have a primary particle diameter of less than (x+y)/2 (nm) is represented by X% by number and the number proportion of inorganic fine particles on the side of large particle diameter which have a primary particle diameter of (x+y)/2 (nm) or more by Y% by number, and have a value of z/x of from 150 to 400 where the volume-average particle diameter of toner particles is represented by z (nm).
However, in this inorganic fine particles, the peak of the primary particle diameter on the side of small particle diameter in the number primary particle diameter distribution is as relatively large as 20 nm or more, and moreover a peak is also present on the side of large particle diameter. Hence, when calculated on the basis of weight, it follows that the large-particle-diameter inorganic fine particles are present in a very large number in respect to the small-particle-diameter inorganic fine particles, bringing about problems on fluidity and chargeability.
Japanese Patent Applications Laid-Open No. H08-36316, No. 2000-56595 and No. 2002-23414 disclose, in a contact transfer assembly in which a bias is applied to a transfer member by a means for applying the bias and a toner held on a latent image bearing member, prepared by externally adding to and mixing in toner particles at least two types of external additives different in average particle diameter, is transferred to a transfer medium, a contact transfer assembly which has defined the relationship between the loose apparent density of the toner and the hardness of the transfer member. However, the respective two types of external additives different in average particle diameter, used here, have separately been hydrophobic-treated, and hence the both differ in their agglomerative properties and readiness of dispersion on toner particle surfaces, and it has been difficult to disperse the both uniformly on the surfaces of toner particles.
Under such a technical background, it is earnestly sought to newly provide novel hydrophobic inorganic fine particles such that the small-particle-diameter inorganic fine particles and the large-particle-diameter inorganic fine particles can readily and uniformly be dispersed on the toner particle surfaces even where they have been hydrophobic-treated.